Answer:
The cyanide ion is a strong nucleophile.
Explanation:
Strong acids
Strong acids like hydrochloric and sulfuric acid dissociate completely in solution.
[tex]\underbrace{\hbox{HCl }}_{\hbox{strong acid}} + {\text{ H$_{2}$O }} \longrightarrow \text{H}_{3}\text{O}^{+}+ \underbrace{\hbox{Cl^{-}}}_{\hbox{weak nucleophile}}[/tex]
Because they are strong acids, their conjugate bases are extremely weak bases/nucleophiles.
Thus, they can protonate the carbonyl oxygen, but the conjugate bases cannot act as nucleophiles.
Weak acids
Weak acids like HCN dissociate only slightly in solution.
[tex]\underbrace{\hbox{HCN}}_{\hbox{weak acid}} + {\text{ H$_{2}$O }} \rightleftharpoons \text{H}_{3}\text{O}^{+}+ \underbrace{\hbox{:CN^{-}}}_{\hbox{strong nucleophile}}[/tex]
Because HCN is a weak acid, its conjugate base is a strong nucleophile.
Thus, it generates relatively few hydronium ions, but the cyanide ion is a strong nucleophile that can attack the partially positive carbon and form the cyanohydrin.
RCH=O + CN⁻ ⟶ RCH(CN)O⁻ ⟶ RCH(CN)OH
As the CN⁻ ions react with the aldehyde or ketone, they are removed from solution.
According to Le Châtelier's Principle, more HCN dissociates to replace the CN⁻ ions, and the reaction goes nearly to completion.
